Vacuum vapor deposited zinc coatings

ABSTRACT

ADHERENT VACUUM VAPOR DEPOSITED ZINC COATINGS ARE PROVIDED FERROUS SUBSTRATES BY INITIALLY VACUUM VAPOR COATING THE SUBSTRATES WITH A FIRST METAL COMPRISING EITHER ALUMINUM OR MAGNESIUM AND THEREAFTER VACUUM VAPOR COATING THE SUBSTRATES WITH ZINC. THE TEMPERATURE OF A SUBSTRATE DURING THE COATING PROCEDURE AND THE ELAPSED TIME BETWEEN APPLICATION OF THE SUCCESSIVE COATINGS ARE CONTROLLED WITHIN CRITICAL LIMITS.

Oct 2.4, 1972 l R. G. RUBIN T 3,700,485

VACUUM VAPOR DEPOSITED ZINC coATINGs Filed April 1o, 1970 INVENTOR ROBERT G. RUBIN BYM ATTORNEY United States Patent O 3,700,485 VACUUM VAPOR DEPOSITED ZINC COATINGS Robert G. Rubin, Pittsburgh, Pa., assiguor to Jones & Laughlin Steel Corporation, Pittsburgh, Pa. Continuation-impart of application Ser. No. 682,354, Nov. 13, 1967. This application Apr. 10, 1970, Ser.

Int. Cl. B44d 1/16 U.S. Cl. 117-71 M 9 Claims ABSTRACT F THE DISCLOSURE Adherent vacuum vapor deposited zinc coatings are provided ferrous substrates by initially vacuum vapor coating the substrates with a rst metal comprising either aluminum or magnesium and thereafter vacuum vapor coating the substrates with zinc. The temperature of a substrate during the coating procedure and the elapsed time between application of the successive coatings are controlled within critical limits.

CROSS REFERENCE TO A RELATED APPLICATION This application is a 4continuation-in-part of my application Ser. No. 682,354, tiled Nov. 13, 1967, now abandoned.

BACKGROUND OF THE INVENTION This invention relates generally to a method for providing adherent vacuum vapor deposited zinc coatings on ferrous substrates, employing an aluminum or magnesium precoat.

In coating a ferrous substrate such as steel strip with a different coating metal by vaporizing the coating metal in an evacuated chamber and causing it to condense on the substrate, it is often difficult to obtain satisfactory adherence of the coating metal to the substrate. Zinc, which is of considerable commercial importance, is a particularly diicult coating metal to control in this respect. By means of the present invention, however, adherent zinc coatings on ferrous substrates are obtained.

It has been my experience that it is less diicult to obtain good adherence of vacuum vapor deposited aluminum and magnesium coatings to steel substrates than it is to obtain equally good adhesion of vacuum vapor deposited zinc coatings to similar substrates. At the same time, I have found that, under certain prescribed conditions, good adherence of vacuum vapor deposited zinc coatings to vacuum vapor deposited aluminum and magnesium coatings can be achieved. Accordingly, the present invention is a method which comprises broadly the steps of vacuum vapor coating a ferrous substrate with a precoat metal comprising aluminum or magnesium and subsequently vapor depositing zinc onto the precoat metal while controlling the temperature of the substrate and the time period between deposition of the two metals within critical limits. As an adjunct of this method, a process for preparing the substrate prior to deposition of the precoating metal in a manner so as to insure adhesion of that metal to the substrate is disclosed.

SUMMARY OF THE INVENTION An object of the present invention is to provide adherent zinc coatings on ferrous substrates. Another object of the invention is to condition ferrous substrates whereby vacuum vapor deposited zinc coatings will adhere thereto. Yet another object of the present invention is to provide adherent aluminum and magnesium coatings on a ferrous substrate by means of which adherent zinc coatings are provided. These and other objects and 3,700,485 Patented Oct. 24, 1972 BRIEF DESCRIPTION OF THE DRAWING In the drawing, a ferrous substrate in the form of a steel strip 1 is passed horizontally through an evacuated chamber 2 from an uncoiling reel 3 to a coiling reel 4. Reel 4 is suitably driven so as to continually move the strip through the chamber at any desired speed. Within chamber 2, positioned below the path of travel of the substrate, are crucibles 5 and 6 which are of approximately the same width as the strip. The precoating metal, either aluminum or magnesium, emanates from crucible 5 in the form of vapor 10. The source material is maintained as a liquid pool 9 Within Crucible 5 and is heated to vaporization by means of an electron beam issuing from electron gun 7. The metal vapor impinges on the undersurface of the substrate as the substrate moves through the evacuated chamber and condenses thereon. In Crucible 6 is maintained a pool of liquid zinc 11 which gives off vapor 12 at its surface. The zinc is heated to vaporization by means of a resistance heater 8. Crucible 6 is positioned downstream of crucible 5 so that the zinc vapor impinges on the undersurface of substrate 1 and condenses thereon over the previously deposited aluminum or magnesium. An electron beam heater 14 is positioned adjacent to and downstream of crucble 6 and is used to assist in controlling the temperature of the substrate in a manner hereinafter described. Positioned above the strip, downstream of each of the crucibles, are temperature measuring devices 15 and 16 for measuring the temperature of the substrate. The evacuated chamber is maintained at a suitable pressure, for example 1.5 10*4 torr, by means of vacuum pumps, not shown. A rotary wire brush 17 is employed between reel 3 and chamber 2 in contact with the underside of strip 1 for abrasively cleaning the strip. The brush is suitably driven in a direction opposite to the direction of travel of the strip so as to have a scouring action on the strip, thereby insuring good cleaning of the strip. Such cleaning insures good adhesion to the strip of the precoating metal.

As the strip leaves uncoiling reel 3 it is abrasively cleaned by wire brush 17, as described, and, thereafter, the strip enters chamber 2 where it first passes over crucible 5. The aluminum or magnesium precoat vapor 10 emanating from crucible 5 is deposited onto the undersurface of the strip, i.e., to a position over and in contact with the strip, as the strip passes over the crucible. Thereafter, the strip passes over Crucible 6 which contains molten zinc 11 and the zinc vapor 12 deposited onto the previously deposited aluminum or magnesium. After being coated with zinc the strip passes over heater 14 which is used to assist in controlling the strip temperature in a manner described below. The strip then passes from the vacuum chamber and is coiled on reel 4. The spacing of crucibles 5 and 6 is correlated with the speed of travel of the strip so that the elapsed time between deposition of the precoat metal and the zinc can be controlled in a desired manner.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS I have discovered that the use of an aluminum or magnesium precoat will enable adherent zinc coatings to be provided on a ferrous substrate only when three operating variables, (1) the substrate temperature during and after precoat metal deposition up until zinc deposition, (2) the substrate temperature during and after zinc deposition,

and (3) the elapsed time between deposition of the two metals, are controlled within prescribed llimits. These variables are herein referred to as initial temperature, final temperature and elapsed time, respectively. Briefly, for any given initial temperature, the longer the elapsed time, the higher must be the final temperature for satisfactory zinc adherence. Thus, final temperature varies directly with elapsed time at given initial temperatures. Further, for any given elapsed time, the higher the initial temperature, the higher must be the final temperature for satisfactory zinc adherence. Thus, final temperature varies directly with initial temperatures at given elapsed times. Finally, for any given final temperature, the longer the elapsed time, the lower must be the initial temperature for satisfactory zinc adherence. Thus, initial temperature various inversely with elapsed time at given final temperatures.

An absolute operating condition of the method of the present invention which is applied in conjunction with the above-noted relationships is the maintenance of the final temperature below the reevaporation temperature of zinc. That is, the final temperature must not be so high as to prevent the zinc from being deposited or to cause the a1- ready deposited zinc to reevaporate from the substrate. At a routine operating vacuum chamber pressure of 1.5 X 10-4 torr, zinc will begin to reevaporate from the substrate at about 500 F. The zinc reevaporation temperature will vary, of course, with the vacuum chamber pressure in a manner as defined by the zinc vapor pressure curve.

It will be understood that because the initial temperature varies inversely with elapsed time at any given final temperature for producing satisfactorily adherent zinc coatings and because some minimum time will necessarily elapse between deposition of the first metal and zinc (the magnitude of the minimum elapsed time depending on the mechanics of the coating system), for any given final temperature there is a maximum or critical initial temperature above which satisfactory zinc adherence cannot be obtained, even if the zinc is deposited after the first metal in that minimum elapsed time. This maximum or critical initial temperature varies directly with the final temperature, and, thus, there is a critical initial temperature which is uniquely related to a final temperature corresponding to the reevaporation temperature of zinc; and since the reevaporation temperature of zinc must not be exceeded, the maximum initial temperature which is related thereto similarly must not be exceeded. This relationship and the relationships between initial temperature, final temperature and elapsed time previously noted will now be considered in more detail with reference to the following table.

RELATIONSHIP BETWEEN INITIAL TEMPERATURE, FINAL TEMPERATURE AND ELAPSED TIME FOR ZINC ADHERENCE USING ALUMINUM PRECOAT Initial Elapsed Final temperatime tempera- Conditions ture F.) (seconds) ture F.)

The above table contains data which defines the various relationships between initial temperature, final temperature and elapsed time for producing satisfactorily adherent zinc coatings on a ferrous substrate using an aluminum precoat. Thus, column 3 of the table lists the maximum elapsed time for satisfactory zinc adherence to an aluminum precoated ferrous substrate when the substrate is controlled at an initial temperature listed in column 2 and a final temperature listed in column 4. For example, when the initial temperature is controlled at 80 F. and the final temperature at 300 F., the maximum elapsed time for satisfactory zine adherence is something less than 2 seconds, while at initial and final temperatures of F. and 450 F., satisfactory zinc adherence is obtained only when the zinc is deposited within 20 seconds of the aluminum.

The data set out in the table was obtained by wire brushing steel strip in air, passing the strip into a vacuum chamber, vapor depositing an aluminum precoat about l0 to 20 microinches thick onto the strip, controlling the strip temperature ata predetermined initial temperature, thereafter, within a predetermined elapsed time, vapor depositing a zinc coating about 0.4 to 1.2 mils thick over the aluminum precoat, controlling the strip temperature at a predetermined final temperature, removing the strip from the vacuum chamber and testing the strip for zinc adherence. The vacuum level of the chamber was maintained at 1.5 X l0"4 torr during all experiments. By varying the initial and final temperatures and the elapsed time, the interrelation of these variables in producing adherent zinc coatings was obtained. Thus, samples controlled at an initial temperature of 80 F., and experiencing elapsed times of 2, 6, 13 and 20 seconds, had to be controlled at final temperatures of at least 300, 350, 400 and 450 F., respectively, in order to obtain satisfactory zinc adherence. In testing for zinc adherence, an Olsen cup test was performed on the coated samples, an adhesive tape was secured to the fractured area of the samples and then rapidly removed. If only a negligible amount of zinc adhered to the tape from the fractured areas, the coating adherence was considered satisfactory.

The fact that the initial temperature varies inversely with elapsed time at any given nal temperature is evidenced by comparing conditions B with conditions E, con ditions C with conditions F, and conditions D with conditions G and H. Thus, at any given final temperature, the highr the initial temperature the shorter must be elapsed time to produce zinc adherence. Similarly, a comparison of conditions A through D or conditions E through G demonstrates that the final temperature varies directly with elapsed time at any given initial temperature for satisfactory zinc adherence so that at a given initial temperature the greater the elapsed time the higher need be the final temperature. It follows then from these two relationships that at any given elapsed time, the higher the initial temperature of the higher need be the final temperature to get the zinc to adhere.

While I do not wish to be Ibound by the following explanation of the reasons for the existence of the aboveexpressed relationships, it appears that the zinc does not adhere when the aluminum is extensively contaminated by oxidation and that, in addition, since the maximum elapsed time for good zinc adhesion declines sharply with higher initial temperatures, the aluminum contamination is predominantly reaction rate controlled rather than controlled by the availability of oxygen at about 1.5 X 10-4 torr pressure. As a result, although the general relationships expressed above will persist 'at any pressure, the absolute values set out in the table hold only at a pressure of 1.5 X104 torr in the same type of atmosphere.

Referring again to the table and specifically to conditions D, G and H thereof, it can be seen that at a final temperature of 450 F., which at a pressure of 1.5 X 10-1 torr is near the reevaporation temperature of zinc, and at initial temperatures of 80, and 200 F., satisfactory zinc adherence can be obtained for elapsed times of up to about 20, 10 and 5 seconds, respectively. Thus, as the initial temperature increases the elapsed time for which satisfactory zine adherence can be obtained decreases. Some minimum time will necessarily elapse between deposition of the first metal and zinc, the magnitude thereof depending on the mechanics of the coating s'ystem and the arrangement of the various components therein. Consequently, there is a minimum or critical initial temperature above which zinc adherence cannot be obtained even if the zinc is deposited after the first metal in that minimum time period. f course, the critical initial temperature varies with the final temperature, and, therefore, there is a critical initial temperature which is uniquely related to a final temperature corresponding to the reevaporation temperature of zinc. It can thus be understood that because the final temperature of the strip must not exceed the reevaporation temperature of zinc, the critical initial temperature uniquely related to the reevaporation temperature similarly must not be exceeded.

In practicing the present invention in a manner as heretofore described with reference to the accompanying figure, the strip temperature will be affected by the abrasive action of the rotary wire brush 17, the heat of condensation of the metal vapor 10, the heat of condensation of the zinc vapor 12 and the heater 14, if employed. In the first instance, the strip temperature during and after deposition of the precoating metal must be controlled so as to be less than the critical initial temperature uniquely related to the revaporation temperature of zinc at the pressure of the vacuum chamber 2. Preferably, the strip temperature will be controlled so as to be substantially below said critical initial temperature. Such control of the strip temperature can be brought about in various ways. For example, the strip can be stored in a cool environment so that when it is placed on reel 3 and subjected to the heating effects of the wire brushing and the condensation of the precoating metal, its initial temperature will fall substantially below said critical initial temperature. In addition, temperature control can be effected by subjecting the strip to varying degrees of abrasive action by wire brush 17, since the greater the abrasive action the greater will the strip be heated; or the strip can be passed through a cooling means after wire brushing so as to dissipate the heat imparted by brushing. Temperature control can also be accomplished by controlling the thickness of the precoating metal deposited on the strip since heating of the strip due to the condensation of the precoating metal thereon will be greater when the amount of metal deposited is greater, and the amount of metal deposited reflects itself in the precoating metal thickness.

Control of the final strip temperature can similarly be accomplished in various ways. Initially, of course, by controlling the initial temperature, the final temperature can be controlled by controlling the thickness of the zinc deposited on the strip. Further, a heating means such asheater 14 can be provided to heat the strip, thereby assisting in controlling the final temperature. The manner of control of the final strip temperature is such, of course, that the final temperature is less than the reevaporation temperature of zinc but greater than the temperature required for satisfactory zinc adherence based on the initial temperature and the elapsed time employed.

In addition to controlling the initial and final strip temperatures, the elapsed time is also controlled. This is accomplished in the illustrated embodiment of the invention by controlling the distance between the crucibles 5 and 6 and the speed of the strip. The maximum elapsed time employed is that time which for satisfactory zinc adherence requires the stripV to be heated to a nal temperature equivalent to the reevaporation temperature of zinc based on the initial strip temperature.

It is preferred to bring the strip into the vacuum chamber at as low a temperature as practical so that the initial temperature, as a result, is also low. As can be understood from the foregoing discussion, satisfactory zinc adherence can be obtained over a wider range of elapsed time and final temperature values when the initial temperature is low.

In applying the aluminum andmagnesium coatings, I have found that satisfactory adhesion of these metals to ferrous substrates results when the substrate is adequately scoured by means of a wire brush and thereafter coated before it can be contaminated. The brushing can be carried out in air and the strip need not be coated with the magnesium or aluminum for thirty minutes or longer depending on the humidity and gaseous impurities in the atmosphere. The particular technique employed in obtaining satisfactory adhesion of the precoating metal is not critical, however, to obtaining satisfactory zinc adhesion; of course, satisfactory zinc adhesion cannot result if the precoating metal does not satisfactorily adhere to the substrate.

The relationships established between initial temperature, final temperature and elapsed time using aluminum as a. precoat are also obtained when a magnesium precoat is employed. The absolute values obtained are different, however.

I claim:

1. A method of producing an adherent zinc coating on a ferrous substrate comprising passing the substrate into an evacuated chamber and while the substrate is in the chamber vapor depositing a first metal selected from the group consisting of aluminum and magnesium onto the substrate, thereafter vapor depositing a coating of zinc onto said first metal, and controlling the initial temperature and final temperature of the substrate and the elapsed time between deposition of the coatings in a manner to produce a zinc coating having satisfactory zinc adherence.

2. The method of claim 1 wherein the first metal deposited is magnesium.

3. The method of claim 1 including the step of cleaning the substrate surface to be coated by wire brushing that surface outside the evacuated chamber.

4. Ihe method of claim 3- wherein the first metal deposited is aluminum.

5. The method of claim 3 wherein the first metal deposited is magnesium.

6. The method of claim 1 wherein the first metal deposited is aluminum.

7. The method of claim 6 including maintaining the pressure within said evacuated chamber at about 1.5 10-4 torr, maintaining the substrate temperature during aluminum deposition and up until zinc deposition below F., depositing the zinc within 20 seconds of depositing the aluminum, and maintaining the final substrate temperature at about 45 0 F.

8. The method of claim 6 including maintaining the pressure within said evacuated chamber at about 1.5 104 torr, maintaining the substrate temperature during aluminum deposition and up until zinc deposition below F., depositing the zinc within 10 seconds of depositing the aluminum, and maintaining the final substrate temperature at about 450 F.

9. The method of claim 6 including maintaining the pressure within said evacuated chamber at about 1.5 104 torr, maintaining the substrate temperature during aluminum deposition and up until zinc deposition below 200 F., depositing the zinc within 5 seconds of depositing the aluminum, and maintaining the final substrate temperature at about 450 F.

References Cited UNITED STATES PATENTS 2,741,016 1f/1956` Roach 29-197 X 2,382,432 8/1945 McManus et al 117-107 3,123,493 3/1964 Brick 117-131 X 2,490,978 12/19'49 Osterheld 29-l96.2 X 2,812,270 11/ 1957 Alexander 117-107 X 3,055,717 9/1962 Schmidt 29-197 X ALFRED L. LEAVITT, Primary Examiner C. K. WEIFFENBACH, Assistant Examiner U.S. Cl. X.R. 

